Pre-impregnation of chips with sulfide-containing solutions accelerates the delignification and improves the selectivity in the subsequent sulfate cooking. The cooking can in this case be carried out at low kappa numbers without impairing the quality of the pulp. The strength characteristics, in particular the tearing strength, of pulp which has been cooked following such impregnation are substantially better. The improvement in the strength characteristics is retained or is even enhanced in the subsequent bleaching.
Pre-impregnation of chips is described extensively in the patent literature. Examples which may be mentioned here are EP-0 527 294, SE-359 331, SE-468 053 and SE-469 078.
However, the previously proposed methods for pre-impregnation of chips do not provide any possibility of controlling certain parameters during different parts of the impregnation, such as the HS.sup.- /OH.sup.- ratio, in order thereby to reduce the attack by the chemicals on the carbohydrates of the hemicellulose and of the cellulose and to reduce the shive content in the pulp, after the cooking, to an even lower level than has hitherto been possible, and such as the temperature, in order thereby to improve the heat economy.
The object of the present invention is to improve the impregnation by creating conditions which are such that certain parameters can be controlled to assume different values during different parts of the impregnation.
The method according to the invention is characterized in that liquid is extracted at a second point situated at the start of the second cocurrent impregnation zone and is made to circulate in an impregnation circulation which empties out at the center of the vessel at a point situated between said first and second points for extraction of liquid so that a free flow of liquid is established from the center of the vessel in a mainly radial direction, and in that said further impregnation liquid, which consists of one or more of the following liquids--black liquor, white liquor, green liquor, liquid from a transfer circulation between the impregnation vessel and a digester, and wash liquor--is supplied to said impregnation circulation for continued impregnation of the fiber material in the second cocurrent impregnation zone. The method according to the invention thus involves a continuous two-stage impregnation in one and the same vessel.
Black liquor which is supplied to said impregnation circulation expediently has a temperature of 120-170.degree. C.
In an expedient embodiment of the invention, in which black liquor is extracted from the digester and is transferred to a plurality of flash cyclones which are connected in series, the black liquor which is supplied through said feeding system is part of the black liquor which is extracted from the digester, or of the effluent from one of said flash cyclones, preferably the last flash cyclone but one. Under the same conditions, black liquor which is supplied to said impregnation circulation can, in the same way, expediently be part of the black liquor which is extracted from the digester, or of the effluent from one of said flash cyclones.
According to the invention, it is advantageous for the impregnation liquids to be chosen, distributed and supplied in such a way that the HS.sup.- /OH.sup.- ratio in the feeding system is as high as possible and expediently higher than in the second impregnation stage.
According to the invention, it is expedient for the temperature in the first cocurrent impregnation zone A to be 100-140.degree. C., preferably 120-130.degree. C., and for the temperature in the second cocurrent impregnation zone B to be 120-160.degree. C., preferably 130-150.degree. C.
According to the invention, it is furthermore expedient for the dwell time of the fiber material in the first cocurrent impregnation zone A to be at least 15 minutes, and for the dwell time in the second cocurrent zone B to be at least 10 minutes.